Most of the time when there is a starter drive to flexplate/flywheel problem it is that the drive is too deep into the starter ring. New or rebuilt starters used to come with a depth gauge which was just a short piece of rod to hold into the bottom of the ring gear groove to check the drive gear tooth depth. I think it was 9/64 but a google search would be in order here. A 9/64 drill stem would work fine. As Jerry stated, remove solenoid so you can engage the drive by hand. I don't know about a GMC but on Chevy's there was a 153 and 168 tooth starter ring so make sure your drive is a match to the ring gear tooth count. On my 37 Buick straight eight with a Bendsten's adapter to the 1963 Chevy and newer bell housing pattern I'm using a 2004R OD automatic and had to use 0.032 in shims (with a vertical mount starter) to get a quiet meshing. Good luck.

Evan